Spin-Selectivity in Photochemistry: A Tool for Organic Synthesis

Abstract

The photochemistry of carbonyl substrates is largely dependent on the spin multiplicity of the excited state. Singlet and triplet excited carbonyls can differ strongly in chemo-, regio-and stereoselectivity. In bimolecular Paternò-Büchi reactions, both excited singlet and triplets states give rise to oxetane formation, albeit with different selectivities and completely different activation parameters. In unimolecular photocyclizations (Norrish-Yang reaction) the triplet state dominates the reaction. For triplet to singlet intersystem crossing at the biradical level, spin-orbit coupling optimizing geometries are crucial. These geometries are different from classical closed-shell interactions and thus give rise to unusual product stereochemistry as well as unusual concentration and temperature dependences.

1,n‐Hydrogen‐Atom Transfer (HAT) Reactions in Which n≠5: An Updated Inventory

“…The selection of appropriate carbonyl compounds enables Norrish type II reaction to generate 1,7‐biradicals 34c. Aspartic derivatives 53.1 a – c were engaged in highly diastereoselective 1,6‐photocyclizations affording 53.3 a,b or 53.4 c in good yields.…”

Electron and hydrogen transfer in organic photochemical reactions

“…Perhaps, the most prominent example is the Norrish–Yang reaction. Particular studies on the relationship between stereochemistry and spin multiplicity or intersystem crossing have been carried out . The reaction was also performed with compounds in confined environments such as zeolites or crystals .…”

Mechanism of the Norrish–Yang Photocyclization Reaction of an Alanine Derivative in the Singlet State: Origin of the Chiral‐Memory Effect

“…The minor trans isomer 3 ( cis / trans 85:15) was also formed with 88 % ee . This reaction is one of the few remarkable examples of the photochemical “chiral‐memory effect” 11. Furthermore, it represents an example of the exploitation of light energy to drive asymmetric synthesis.…”